Foamable polycarbonate compositions, articles and methods

ABSTRACT

A foamable thermoplastic composition comprising in admixture (a) a polycarbonate resin, alone, or with (b) a polyester, (c) a minor amount of a foaming agent and (d) a minor amount of an inorganic non-fibrous particulate nucleating agent, alone, or with a particulate organic polymeric material, wherein (d) has an aspect ratio of about 1 and a small particle size.

This invention relates to foamable thermoplastic compositions comprisingin admixture (a) a polycarbonate resin, alone, or in combination with(b) a polyester resin, (c) a minor amount of a chemical foaming agentand (d) a minor amount of a foam nucleating agent. Also provided arearticles made by foaming such compositions.

BACKGROUND OF THE INVENTION

High molecular weight aromatic polycarbonates and blends thereof withlinear polyesters are known, and these can include finely dividedparticulate fillers having a low aspect ratio, such as silica, andpolymeric particles, see, e.g., commonly assigned U.S. patentapplications Ser. Nos. 553,172, 553,173, and 553,175, filed Nov. 18,1983, Richard B. Allen.

Also, it is known to add a foaming agent to thermoplastics as disclosedin U.S. Pat. Nos. 3,781,233, 3,799,954, 4,097,421, 4,097,425, 4,280,005,and 4,351,911.

In U.S. Pat. Nos. 4,280,005 and 4,351,911, it is reported that when afoaming agent is added directly to a polyester, highly irregular foamingoccurs which results in an inferior product in that the product containsvoids and sink marks. The product is also warped. However, when thefoaming agent is added to a polyester containing a filler such asfibrous glass, minerals or mixtures thereof in an amount of at least 5percent, a foamed product is produced having a rigid cellular corewithin a solid integral skin. Fibrous glass is a filler with a highaspect ratio, i.e., a length to diameter ratio of greater than 1.0,substantially so.

When a foaming agent is added directly to a polycarbonate, the materialwill not form a foam when injected into a cavity, the polymer collectinginstead as solid masses in the mold. However, it is known to use glassfibers to aid in the formation of gas bubbles which ultimately formcells, and foams. Glass fibers do not, however, produce an entirelyuniform, void-free structure with polycarbonates and the glass fibersproduce severe and adverse reduction in impact strength, especially atlow temperatures and an unsmooth surface appearance, especially ifrecent techniques, such as counterpressure foam molding, are used toproduce the parts. High impact strengths are important becauseapplications of polycarbonate structural foams have begun to expandbeyond the traditional business equipment and computer housings intouses such as automobile load floors, seats, truck battery boxes, and thelike, and low temperature impact strength, especially with the latterare critical.

It has now been discovered that structural foams comprising aromaticpolycarbonate resins and related resins can be nucleated without thepresence of glass fiber to yield foams that are superior in impactstrength. Nucleating agents that can be used include but are not limitedto amorphous silica, precipitated silica, titanium dioxide, particulateimpact modifiers, foamed silica, dispersible polytetrafluoroethyleneparticles, and the like. The use of surfactants is also preferred tohelp bubble formation and to improve nucleating agent dispersion. Mostpreferably, the nucleating agent should be sub-micron in size and itshould be coated with a hydrophobic coating. Surfactants which can beused include but are not limited to silicones, polyglycol ethers,fluorocarbons, fatty acid derivatives, and the like.

The compositions of the instant invention can be molded, for example, instandard injection molding machines, to produce a variety of foam moldedproducts having good surface appearance, uniform cell structure, andexceptional impact strength.

SUMMARY OF THE INVENTION

According to the present invention, there are provided foamablethermoplastic compositions, comprising

(a) an aromatic polycarbonate, an aromatic polyester carbonate, anaromatic dihydric phenol sulfone carbonate, or a mixture of any of theforegoing, alone, or in combination with

(b) a polyester resin comprising units of an aliphatic diol, acycloaliphatic diol, or a mixture of said diols and an aromatic diacidor a cycloaliphatic diacid,

(c) a minor effective amount of a foaming agent; and

(d) a minor effective amount of a particulate foam nucleating agentsubstantially insoluble in the composition and having an aspect ratio ofnot greater than about 1.0, the amounts of (c) and (d) being sufficientto provide a rigid, substantially void-free, substantially uniformcellular core within a solid integral skin.

In preferred features of the invention the compositions will besubstantially free, that is, will contain 0 or no more than about 2% byweight of fillers with an aspect ratio of greater than about 1.0, e.g.,talc platelets or glass fibers, etc. Other preferred features are thosein which component (a) comprises from about 99.97 to about 50.0 parts byweight; component (b) comprises from about 0 to about 50 parts byweight; component (c) comprises from about 0.02 to about 2.0 parts byweight; and component (d) comprises from about 0.01 to about 40 parts byweight based upon the total weight of components (a), (b), (c) and (d)combined.

The thermoplastic compositions of the present invention may also includepigments, such as carbon black, impact modifiers, flow promoters and thelike, as well as flame retardant agents.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by reference to the drawing inwhich

FIG. 1 is a photomicrograph at 30X magnification of a cross section ofglass fiber filled polycarbonate foamed according to the prior art,showing non uniform cells and numerous voids; and

FIG. 2 is a photomicrograph at 30X magnification of a cross section of anon-glass filled nucleated polycarbonate foamed according to the presentinvention, showing substantially uniform cells and substantially novoids.

DETAILED DESCRIPTION OF THE INVENTION

Polycarbonate homopolymers useful in this invention are especiallyaromatic polycarbonates. These can be made by those skilled in the artor obtained from various commercial sources. They may be prepared byreacting dihydric phenol with a carbonate precursor, such as phosgene, ahaloformate or a carbonate ester. Typically, they will have recurringstructural units of the formula: ##STR1## wherein A is a divalentaromatic radical of the dihydric phenol employed in the polymerproducing reaction. Preferably, the aromatic carbonate polymers have anintrinsic viscosity ranging from 0.30 to 1.0 dl./g. (measured inmethylene chloride at 25° C.) By dihydric phenols is meant mononuclearor polynuclear aromatic compounds containing two hydroxy radicals, eachof which is attached to a carbon atom of an aromatic nucleus. Typicaldihydric phenols include 2,2-bis-(4-hydroxy-phenyl)propane;2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)propane, 4,4'-dihydroxydiphenylether, bis(2-hydroxyphenyl)methane, mixtures thereof and the like. Thepreferred aromatic carbonate polymer is a homopolymer derived from2,2-bis(4-hydroxyphenyl)propane(bisphenol-A).

Poly(ester-carbonates) for use in the invention are known and can beobtained commercially. Generally, they are copolyesters comprisingrecurring carbonate groups: ##STR2## carboxylate groups: ##STR3##aromatic carbocyclic groups in the linear polymer chain, in which atleast some of the carboxylate groups and at least some of the carbonategroups are bonded directly to ring carbon atoms of the aromaticcarbocyclic groups. These poly(ester-carbonate) copolymers, in general,are prepared by reacting a difunctional carboxylic acid, such asphthalic acid, isophthalic acid, terephthalic acid, homophthalic acid,o-, m-, and p-phenylenediacetic acid, the polynuclear aromatic acids,such as diphenic acid, 1,4-naphthalic acid, mixtures of any of theforegoing, and the like, with a dihydric phenol and a carbonateprecursor, of the types described above. A particularly useful polyestercarbonate is derived from bisphenol-A, isophthalic acid, terephthalicacid, or a mixture of isophthalic acid and terephthalic acid, or thereactive derivatives of these acids such as terephthaloyl dichloride,isophthaloyl dichloride, or a mixture thereof, and phosgene. The molarproportions of dihydroxy diaryl units to benzenedicarboxylate units tocarbonate units can range from 1:0.2-1.00:0.80-0.00 and the molar rangeof terephthalate units to isophthalate units can range from 99:1 to 1:99in this preferred family of resins. When the molar proportion ofcarbonate units is 0, the resin is a wholly aromatic polyester. SeeRobeson, U.S. Pat. No. 4,324,869.

The aromatic dihydric phenol sulfone resins used in this invention are afamily of resins which can be made by those skilled in this art. Forexample, homopolymers of dihydric phenol, and a dihydroxydiphenylsulfone and a carbonate precursor can be prepared, as well as copolymersof a dihydric phenol and a carbonate precursor can be made according tothe description in Schnell, et al., U.S. Pat. No. 3,271,367. A preferredmaterial is made by polymerizing bis-(3,5-dimethyl-4-hydroxy phenyl)sulfone, alone, or especially in combination with bisphenol-A, withphosgene or a phosgene precursor, in accordance with the description inFox, U.S. Pat. No. 3,737,409. Especially preferred is a coplymer made byreacting 1-99, preferably 40-99 wt. percent of the sulfone, 99 to 1,preferably 60 to 1 wt. percent of the bisphenol with phosgene.

Polyesters suitable for use herein are derived from an aliphatic,aliphatic ether or cycloaliphatic diol, or mixtures thereof, preferablycontaining from about 2 to about 10 carbon atoms, and one or morearomatic or cycloaliphatic dicarboxylic acids. Preferred polyesters arederived from an aliphatic diol and an aromatic dicarboxylic acid havingrepeating units of the following general formula: ##STR4## wherein n isan integer of from 2 to 4. The most preferred polyester is poly(ethyleneterephthalate).

Also contemplated herein are the above polyesters with additionalamounts of polyols and/or acids in the amounts of from 0.5 to 50 wt.percent based on the total composition. The acids can be aliphatic orcycloaliphatic with the number of carbon atoms ranging from 2 to 20.Likewise, the glycols can be cyloaliphatic or aliphatic with the numberof carbon atoms covering the same range. Polyalkylene ether glycols canalso be used where the alkylene portion has from 2 to 10 carbon atomsand the entire glycol portion varies in molecular weight from 100 to10,000. All such polyesters can be made following the teachings of, forexample, U.S. Pat. Nos. 2,465,319 and 3,047,539.

The polyesters which are derived from a cycloaliphatic diol and anaromatic dicarboxylic acid are prepared, for example, by condensingeither the cis- or trans-isomer (or mixtures thereof) of, for example,1,4-cyclohexanedimethanol with an aromatic dicarboxylic acid so as toproduce a polyester having recurring units of the following formula :##STR5## wherein the cyclohexane ring is selected from the cis- andtrans-isomers thereof and R represents an aryl or cycloaliphatic radicalcontaining 6 to 20 carbon atoms and which is the decarboxylated residuederived from an aromatic dicarboxylic acid.

Examples of aromatic dicarboxylic acids represented by thedecarboxylated residue R are isophthalic or terephthalic acid,1,2-di(p-carboxyphenyl)ethane, 4,4'-dicarboxydiphenyl ether, etc., andmixtures of these. Acids containing fused rings can also be present,such as in 1,4- or 1,5-napthalenedicarboxylic acids. Also contemplatedare cycloaliphatic diacids, such as cyclohexane dicarboxylic acid. Thepreferred dicarboxylic acids are terephthalic acid or a mixture ofterephthalic and isophthalic acids.

Another preferred polyester may be derived from the reaction of eitherthe cis- or trans-isomer (or a mixture thereof) of1,4-cyclohexanedimethanol with a mixture of isophthalic and terephthalicacids. Such a polyester would have repeating units of the formula:##STR6##

Still another preferred polyester is a copolyester derived from acyclohexane dimethanol, an alkylene glycol and an aromatic dicarboxylicacid. These copolyesters are prepared by condensing either the cis- ortrans-isomer (or mixtures thereof) of, for example,1,4-cyclohexane-dimethanol and an alkylene glycol with an aromaticdicarboxylic acid so as to produce a copolyester having units of thefollowing formula: ##STR7## wherein the cyclohexane ring is selectedfrom the cis- and trans-isomers thereof, R is as previously defined, nis an integer of 2 to 10, the x units comprise from about 1 to about 99percent by weight, and the y units comprise from about 99 to about 1percent by weight.

Such a preferred copolyester may be derived from the reaction of eitherthe cis- or trans-isomer (or mixtures thereof) of1,4-cyclohexanedimethanol and ethylene glycol with terephthalic acid ina molar ratio of 80:20:100. These copolyesters have repeating units ofthe following formula: ##STR8## wherein x and y are as previouslydefined.

The polyesters described herein are either commercially available orthey can be produced by methods known in the art, including those setforth in U.S. Pat. No. 2,801,466.

The polyesters employed in the practice of this invention will usuallyhave an intrinsic viscosity of from about 0.4 to about 2.0 dl./g., asmeasured in a 60:40 phenol:tetrachloroethane mixture, or similar solventat 23°-30° C.

The relative amounts of the polymers can and usually do vary widely inthe blend, with particular amounts depending on specific requirementsand the nature of the polymers being employed. Proportions can range,for example, between 99:1 and 1:99, based on 100 parts by weight of thetwo polymers together. Best amounts in a given instance will be readilydeterminable by those skilled in the art.

For those embodiments of the invention having flame retardantproperties, conventional additives can be used in conventional amounts.For example, sulfonate salts in small amounts can be used, such as asulfonate salt, e.g., potassium benzenesulfonic acid at least 0.1, andpreferably 0.4 parts per hundred by weight of polycarbonate, or aromatichalogen compounds, such as tetrabromobisphenol A homopolymers orcopolymers or oligomers, alone, or with a suitable synergist, such asantimony oxide, can be used in effective amounts to provide flameresistance, e.g., about 2-12 parts by weight of bromine per 100 parts byweight of flammable resin content.

The foaming agents (c) of the present invention can be selected fromchemicals containing decomposable groups such as azo, N-nitroso,carboxylate, carbonate, heterocyclic nitrogen containing and sulfonylhydrazide groups. Generally, they are solid materials that liberategas(es) when heated by means of a chemical reaction or decomposition.Representative compounds include azodicarbonamide,dinitrosopentamethylene tetramethylene tetramine,p,p'-oxy-bis(benzenesulfonyl)-hydrazide, benzene-1,3-disulfonylhydrazide, azo-bis-(-isobutyronitrile), biuret and urea. Thedihydrooxadiazinones of U.S. Pat. No. 4,097,425 are members of anespecially preferred family of foaming agents. The foaming agent may beadded to the polymer in several different ways which are known to thoseskilled in the art, for example, by adding the solid powder, liquid orgaseous agents directly to the resin in the extruder while the resin isin the molten state to obtain uniform dispersal of the agent in themolten plastic. The temperature and pressures to which the foamablecomposition of the invention are subjected to provide a foamedpolyester/polycarbonate composition will vary within a wide range,depending upon the amount and type of foaming agent that is used. Thefoaming agents may be used in amounts of from about 0.01 to about 2.5parts by weight and preferably from about 0.02 to 2.0 parts by weightper 100 parts by weight of the total composition (resin). The preferredfoaming agents are dinitrosopentamethylene tetramine, p-toluene sulfonylsemicarbazide, 5-phenyltetrazole, calcium oxalate,trihydrazino-s-triazine, 5-phenyl-3,6-dihydro-1,3,4-oxadiazin-2-one,3,6-dihydro-5,6-diphenyl- 1,3,4-oxadiazin-2-one and mixtures of any ofthem.

The purpose of foam nucleating agent (d) is to provide a surface forbubble formation. This is analogous to the use of "boiling chips" tomoderate bubble formation and to prevent "bumping" in boiling liquids.Numerous attempts to form foams from the same resins (a), alone, or with(b), without nucleating agent were unsuccessful. Although glass fibersare better than nothing, they ruin impact properties, as will be seen.The retention of impact properties requires judicious selection ofparticle size, shape, and surface polarity, but the most importantcharacteristic appears to be aspect ratio, that is the dimension in onedirection divided by another. When the length divided by diameter isequal to about 1.0 in aspect ratio, the impact strength is excellent. Ifthe aspect ratio of the foam nucleating agent is greater than about 1.0,e.g., 1.5, the impact strength drops, and if it is significantlygreater, impact strength as measured by standard test methods is verypoor, especially at low temperatures, e.g., -40° C. As has beenmentioned before, also, high aspect ratio fillers, e.g., glass fiberswill show up as surface imperfections, especially if counterpressuremolding is used, but low aspect foam nucleating agents, especially insub-micron sizes, do not detract from surface smoothness.

Although numerous inorganic, non-fibrous particulate compounds can beemployed for use as foam nucleating agents according to this invention,their selection by those skilled in this art is a simple matter. Thecompositions can be formulated and foam molded in standard injectionmolding machines into workpieces suitable for measuring cell structureand flow and ultimately unnotched Izod impact strength. The bestformulations can then, if desired, be scaled up for molding into panelsand then tested for drop ball impact strength, which provides usefulinformation on resistance to ductile impact. The latter test is the mostsensitive to fine formulation changes and has the added advantage ofbeing closest to actual end use requirements.

As has been mentioned, it has been discovered that satisfactory resultsare achievable with siliceous or non-siliceous inorganic patriculatematerial, alone, or together with an organic polymeric particulatematerial. Illustratively, the inorganic material can be amorphoussilica, precipitated silica, fumed silica, and titanium dioxide, and theorganic polymers can be acrylate copolymers, such as those described inU.S. Pat. No. 4,320,212 (Liu); dispersible polytetrafluoroethyleneparticles; and polyphenylene ether(oxide) resin particles such asdescribed in U.S. Pat. Nos. 3,306,874 and 3,306,875 (Hay), as well as inU.S. Pat. Nos. 3,257,357 and 3,257,358 (Stamatoff).

In the present invention, it is preferred to use silica as component(d). This is readily available in a variety of forms with suitableparticle sizes, aspect ratios and surface polarities. Although a widevariety of silicas are used in plastics, for component (c) herein, onlya synthetic type will be used. These differ from silicas of mineraltype, which are mainly used in the form of quartz and glass as fillers.Amorphous, i.e., non-crystalline silicas on the other hand, aretypically used in generally small amounts, to achieve special effects.They are made by well known processes, such as one of the pyrogenicprocesses, i.e., the AEROSIL process in which silicon tetrachloride ismixed with hydrogen and oxygen at 1000° C. or higher, or the electricarc process in which quartz and coke are heated at 1500° C. or above andthe SiO and CO which are formed thereby are further reacted withatmospheric oxygen. Wet proccesses are also useful to prepare silicassuitable for use herein, i.e., a precipitation process in which sodiumwater glass and sulfuric acid are stirred and silica precipitates, or ahydrothermal process, in which quartz is heated with lime under pressureat 180° C., and the calcium silicate is mixed with hydrochloric acid. Inall cases, the silica is obtained in a highly dispersed, i.e., veryfinely divided form. They are not crystalline, but are amorphous, asdetermined by x-rays. Silicas suitable for the present method areavailable from a number of commercial sources. The most importantcharacteristics appear to be average primary particle size, which ismeasured in conventional ways, and expressed in microns (or nanometers,nm); compacted apparent density, measured according to Standard TestMethod DIN 53 194 or lSO 787-11; and expressed in g./1., and pH-value,measured according to Standard Test Method DIN 53 200 or ASTM D 1208-65,and expressed in pH units measured on a 5% aqueous suspension. Inaccordance with conventional practice, the silicas can be surfacetreated with silanes, silicones, and the like. In Table 1 are listed anumber of commercial sources for suitable silicas. The list is notexhaustive but is provided as an aid to understanding and practicing theinvention:

                  TABLE 1                                                         ______________________________________                                        Suitable Commercial Sources of Silicas                                                                      Average                                                                       Agglom-                                                                       erated Surface                                  Manufac-                                                                              Trade                 Size,  Area                                     turer   Name       Type       Microns                                                                              m.sup.2 /g                               ______________________________________                                        CABOT.sup.a                                                                           CABOSIL    Hydrophilic                                                                              2.5    250                                              M-5        Fumed                                                      CABOT   CABOSIL    Hydrophobic                                                                              0.014  70 + 15                                          N70-TS     Fumed                                                      CABOT   CABOSIL    Hydrophilic                                                                              0.008  325 + 25                                         HS-5       Fumed                                                      DAVI-   SYLOID     Synthetic  2.5    250                                      SON.sup.b                                                                             234        Amorphous                                                  DAVISON            Silica     140    --                                                          Gel                                                        IMSIL.sup.c                                                                           IMSIL      Amorphous  1.8     2                                               A108                                                                  IMSIL   A108H      Amorphous, 1.8     2                                                          surface                                                                       treated                                                    TULCO.sup.d                                                                           TULLANOX   Hydrophobic,                                                                             0.018  140 + 15                                         A-50       fumed                                                      ______________________________________                                         .sup.a Cabot Corp., Boston, Ma                                                .sup.b Davison Chemical Div., W.R. Grace & Co., Baltimore, MD 21203           .sup.c Illinois Minerals Co., Cairo, IL 62914                                 .sup.d Tulco Co., Boston, Ma                                             

In preferred embodiments, the foam nucleating agent (d) will carry asurface coating sufficient to render it hydrophobic. Hydrophobic surfacetreated particulate agents suitable for use herein are commerciallyavailable, such as the material known as CabOSil N-70-TS listed in Table1.

As has been mentioned, component (d) can be used in conjunction with anorganic polymer or polymers, for example, acrylate copolymers,polyphenylene ethers or polytetrafluoroethylenes.

The preferred acrylate polymers are acrylate-based core-shellmulti-phase composite interpolymer resins. More particularly, theacrylate-based core-shell multi-phase composite interpolymer resin is acore-shell interpolymer comprising about 25 to 95 percent by weight of afirst elastomeric phase and about 75 to 5 percent by weight of a finalrigid thermoplastic shell phase. One or more intermediate phases areoptional, for example, a middle stage polymerized from about 75 to 100percent by weight styrene.

The first stage or core of the multiphase composite interpolymercomponent is polymerized from about 75 to 99.8 weight percent C₁ to C₆alkyl acrylate resulting in an acrylic rubber core having a T_(g) belowabout 10° C. and crosslinked with 0.1 to 5 weight percent crosslinkingmonomer and further containing 0.1 to 5 percent by weight graft-linkingmonomer. The preferred alkyl acrylate is butyl acrylate. Thecrosslinking monomer is a polyethylenically unsaturated monomer having aplurality of addition polymerizable reaction groups all of whichpolymerize at substantially the same rate of reaction. Suitablecrosslinking monomers include polyacrylic and poly methacrylic esters ofpolyols such as butylene diacrylate and dimethacrylate, trimethylolpropane trimethacrylate, and the like, di- and tri-vinyl benzene, vinylacrylate and methacrylate, and the like. The preferred crosslinkingmonomer is butylene diacrylate.

The graftlinking monomer is a polyethylenically unsaturated monomerhaving a plurality of addition polymerizable reactive groups, at leastone of which polymerizes at substantially different rate ofpolymerization from at least one other of said reactive groups. Thefunction of the graftlinking monomer is to provide a residual level ofunsaturation in the elastomeric phase, particularly in the latter stagesof polymerization, and consequently, at or near the surface of theelastomer particles. When the rigid thermoplastic shell phase issubsequently polymerized at the surface of the elastomer, the residualunsaturated, addition-polymerizable reactive groups contributed by thegraft-linking monomer participates in the subsequent reaction so that atleast a portion of the rigid shell phase is chemically attached to thesurface of the elastomer.

Among the effective graftlinking monomers are allyl group-containingmonomers of allyl esters of ethylenically unsaturated acids, such asallyl acrylate, allyl methacrylate, diallyl maleate, diallyl fumarate,diallyl itaconate, allyl acid maleate, allyl acid fumarate, and allylacid itaconate. Somewhat less preferred are the diallyl esters ofpolycarboxylic acids which do not contain polymerizable unsaturation.The preferred graftlinking monomers are allyl methacrylate and diallylmaleate. A most preferred interpolymer has only two stages, the firststage comprising about 60 to 95 percent by weight of the interpolylmerand being polymerized from a monomer system comprising 95 to 99.8 weightpercent of n-butyl acrylate, 0-2.5 weight percent of butylene diacrylateas cross-linking agent, 0.1 to 2.5 percent by weight allyl methacrylateor diallyl maleate as graftlinking agent with a final stage polymerizedfrom about 60 to 100 percent by weight methyl methacrylate. A preferredtwo stage interpolymer of this type is commercially available under thetradename, ACRYLOID KM 330, from Rohm & Haas Chemical Company.

The final or shell stage monomer system can be comprised of C₁ to C₁₆methacrylate, styrene, acrylonitrile, alkyl acrylates, allylmethacrylate, diallyl methacrylate, and the like, as long as the overallT_(g) system is at least 20° C. Preferably, the final stage monomersystem is at least 50 weight percent C₁ to C₄ alkyl methacrylate. It isfurther preferred that the final stage polymer be free of units whichtend to degrade poly(alkylene terephthalates), for example acid,hydroxyl, amino, and amide groups.

The multiphase composite interpolymers are prepared sequentially byemulsion polymerization techniques wherein each successive outer stagecoats the previous stage polymer. By way of illustration, the monomericC₁ -C₆ acrylate, the cross-linking monomer and the graft-linking monomerare copolymerized in water in the presence of a free-radical generatingcatalyst and a polymerization regulator which serves as a chain transferagent, at a temperature on the order of from 15° C. to 80° C. The firstelastomeric phase is formed in situ to provide a latex of the corecopolymer.

Thereafter, the second rigid thermoplastic phase monomers are added andare emulsion polymerized with the core-copolymer latex to form theinterpolymers. A more detailed description of the preparation of theacrylate-based interpolymers for use herein can be found in U.S. Pat.Nos. 4,034,013 and 4,096,202, both being specifically incorporatedherein by reference.

Preferred polyphenylene ether resins are homopolymers or copolymershaving units of the formula: ##STR9## in which Q, Q', Q" and Q"' are,independently, selected from the group consisting of hydrogen, halogen,hydrocarbon radicals, halohydrocarbon radicals, hydrocarbonoxy radicals,and halohydrocarbonoxy radicals; and n represents the total number ofmonomer units and is an integer of at least about 20, and more usuallyat least 50.

The polyphenylene ether resins are, in general, self-condensationproducts of monohydric, monocyclic phenols produced by reacting thephenols with oxygen in the presence of complex metal catalysts, with themolecular weight being controlled by the reaction time, longer timesproviding a higher average number of repeating units. Particularprocedures are known to those skilled in the art and are described inthe patent literature, including the Hay and Stamatoff patents mentionedabove.

Suitable phenolic monomers include but are not limited to:2,6-dimethylphenol; 2,6-diethylphenol; 2,6-dibutylphenol;2,6-dilaurylphenol; 2,6-dipropylphenol; 2,6-diphenylphenol;2-methyl-6-ethylphenol; 2-methyl-6-cyclohexylphenol;2-methyl-6-tolylphenol; 2-methyl-6-methoxyphenol;2-methyl-6-butylphenol; 2,6-dimethoxyphenol; 2,3,6-trimethylphenol;2,3,5,6-tetramethylphenol; and 2,6-diethoxyphenol.

Some of the polymers which can be produced and which are within theabove formula are: poly(2,6-dilauryl-1,4-phenylene)ether;poly(2,6-diphenyl-1,4-phenylene)ether;poly(2,6-dimethoxy-1,4-phenylene)ether;poly(2,6-diethoxy-1,4-phenylene)ether;poly(2-methoxy-6-ethoxy-1,4-phenylene)ether;poly(2-ethyl-6-stearyloxy-1,4-phenylene)ether;poly(2,6-dichloro-1,4-phenylene) ether;poly(2-methyl-6-phenyl-1,4-phenylene)ether;poly(2,6-dibenzyl-1,4-phenylene)ether; poly(2-ethoxy-1,4-phenylene)ether; poly(2-chloro-1,4-phenylene)ether;poly(2,6-dibromo-1,4-phenylene)ether; and the like.

Also included within the above formula are copolymers prepared frommixtures of phenolic monomers. Special mention is made of those based onthe reaction of 2,6-dimethylphenol with other phenols, for example, with2,3,6-trimethylphenol or 2-methyl-6-butylphenol, to produce thecorresponding copolymer, for example, poly(2,6-dimethyl-co-2,3,6-trimethylphenol),poly(2,6-dimethyl-co-2-methyl-6-butylphenol), and so forth.

Especially preferred for use in this invention are homopolymers havingalkyl substituents in the two positions ortho to the oxygen atom, thatis, those of the above formula in which Q and Q' are alkyl andparticularly alkyl having from 1 to 4 carbon atoms. Most preferred ispoly(2,6-dimethyl-1,4-phenylene ether).

In other embodiments, agents for helping particulate dispersion and toreduce surface tension will be included. These generally will benon-ionic surfactants, and will typically be, for example, silicones,polyglycol ethers, fluorocarbons, fatty acid derivatives, and manyothers. Amounts of from 0.01 to about 5.0 parts by weight per 100 partsby weight of composition will be used.

In some embodiments, the compositions of the present invention willinclude a modifier to increase the flow of resin during molding. Manymaterials are suitable for this purpose, but special mention is made ofparaffin waxes, fatty acid esters, polyethylene waxes or mixtures.Suitable commercially available materials will be illustrated in theexamples, and they can be used in a wide concentration range, preferably0.1 to 5 parts by weight per 100 parts by weight of total composition.

In other embodiments, the compositions present invention can include aneffective amount of (f) a modifier to increase the impact strength ofthe composition. These can comprise organic polymeric materialsgenerally used to enhance the impact strength of thermoplastics.Typically, they can be a block copolymer of butadiene and styrene, ablock copolymer of polycarbonate and polysiloxane, linear low densitypolyolefins and the like. They will be used in conventional amounts,e.g., from 0.5 to 20 parts by weight, preferably from 2 to 10 parts byweight, per 100 parts of total composition.

The relative amounts of components used in the method of the presentinvention can vary widely. For example, the aromatic carbonate polymercomponent (a) can comprise from 100 to 1 parts by weight and polyesterresin component (b) can comprise correspondingly from 0 to 99 parts byweight. Effective amounts of the other additives have been discussedabove. In any event, one will use the lowest amount to obtain thedesired result, since there is the least tendency to adversely affectphysical properties in so-doing.

The resins and other ingredients may be prepared by any of the wellknown methods outlined above. They may be blended with one another atroom temperature. The blend consisting of the polycarbonate, anyoptional resin, the foam nucleating agent, any impact modifier, etc.,are fed into an extruder at a temperature of 480° F. to about 550° F.The extrudate is then comminuted into pellets or other suitable shapes.The decomposable chemical foaming agent can be dry tumbled with theresin blend, or it can previously have been added, e.g., as aconcentrate in polycarbonate resin. This mixture is then fed into aconventional molding machine. The molding temperature may be from about500° F. to about 580° F. with the mold temperature being from about 100°F. to 250° F., preferably from about 140° F. to about 200° F.

The foamable composition may be handled in any conventional manneremployed for the fabrication or manipulation of thermoplastics such aslow and high pressure injection molding to provide thermoplasticproducts which have uniform cell structure, smooth surfaces, highimpact, modulus and tensile strength, improved flow and cycle time.

The composition may contain additional materials such as pigments anddyes, stabilizers, antioxidants, mold-release agents, ultravioletstabilizers, and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are set forth herein to illustrate in more detailthe preferred embodiments and to illustrate more clearly the principleand practice of this invention to those skilled in the art. Unlessotherwise specified, where parts or percents are mentioned, they areparts or percents by weight.

Formulations were compounded by dry blending the components followed byextrusion on a vented, one inch single screw Haake Buchler extruder witha length/ diameter ratio of 25. Generally, the polycarbonate powder wasnot dried prior to extrusion. The extrusion parameters were as follows:

    ______________________________________                                        Extrusion conditions                                                                             Actual     Set                                             ______________________________________                                        Zone 1             285° C.                                                                           285° C.                                  Zone 2             290° C.                                                                           290° C.                                  Zone 3             295° C.                                                                           295° C.                                  Die                295° C.                                                                           295° C.                                  Melt               296° C.                                                                           --                                              Pressure           350 psi    --                                              RPM                50-150     --                                              Amps               20         --                                              ______________________________________                                    

Prior to molding, the compounded resin was dry blended with the blowingagent at a 5/95 blowing agent resin concentrate/polycarbonate resinratio. The blowing agent resin concentrate contains 5 percent by weightof the blowing agent in poly(bisphenol-A carbonate). The dry blend wasfurther dried at 110° C. for 4 hours prior to molding. After drying, thematerial was foam molded to a density of 0.90 g./cc. into Izod impactbars (5"×1/2"×1/2") and drop ball impact plaques (51/2"×11"×1/4").Molding was done in a 101/2 oz. Reed molding press utilizing about 15%and about 80% of shot capacity for Izod bars and plaques respectively.Molding parameters were as follows:

    ______________________________________                                        MOLDING CONDITIONS                                                            ______________________________________                                        Nozzle Temp., °F.                                                                            550                                                     Front Temp., °F.                                                                             580                                                     Center Temp., °F.                                                                            580                                                     Rear Temp., °F.                                                                              480                                                     Mold Temp., °F.                                                                              180                                                     Injection: Hold       100     psi                                             Std.                  0       psi                                             Fast                  700     psi                                             Cure Time             55      sec.                                            Clamp Open            1.0     sec.                                            Cycle Time            62      sec.                                            Shot Time             0.5-1.0 sec.                                            Amt. Blowing Agent Concentrate                                                                      5%      by weight                                       ______________________________________                                    

For testing, the procedure of American Society of Testing Materials(ASTM D-256) was used on unnotched Izod bars. Drop ball impact testingwas done according to ASTM D-2444, slightly modified. Particle size andsurface area measurements on the nucleating agents were obtained fromthe suppliers thereof, or by optical microscopy when not so available.Intrinsic viscosities were measured at 30° C. in CHCl₃, unless otherwisenoted.

EXAMPLES 1-8

Compositions comprising an aromatic polycarbonate resin (GeneralElectric Company, LEXAN 100), a chemical blowing agent, and siliceousfoam nucleating agents having an aspect ratio of about 1.0 wereformulated, foam molded and tested. For comparison purposes, acomposition having a siliceous nucleating agent with aspect ratiosignificantly greater than 1, fibrous glass, was molded and tested. Theformulations used and the results obtained are set forth in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Foamed Compositions of Polycarbonate                                          and Silica                                                                                   Example                                                                       A*  1   2   3   4   5   6   7   8                              __________________________________________________________________________    Composition (parts by weight)                                                 Poly(bisphenol-A carbonate)                                                                  100 100 100 100 100 100 100 100 100                            5-phenyl-3,6-dihydro-1,3,4-                                                                  0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                           oxadiazin-2-one blowing agent                                                 silica, gel.sup.a                                                                            --  0.5 --  --  --  --  --  --  --                             silica, hydrophilic, fumed.sup.b                                                             --  --  0.5 --  --  --  --  --  --                             silica, granular gel.sup.c                                                                   --  --  --  0.5 --  --  --  --  --                             silica, hydrophobic, fumed.sup.d                                                             --  --  --  --  0.5 --  --  --  --                             silica, amorphous.sup.e                                                                      --  --  --  --  --  0.5 --  --  --                             silica, amorphous, surface                                                                   --  --  --  --  --  --  0.5 --  --                             treated.sup.f                                                                 silica, hydrophilic, fumed.sup.g                                                             --  --  --  --  --  --  --  0.5 --                             silica, hydrophobic, fumed.sup.h                                                             --  --  --  --  --  --  --  --  0.5                            Glass, fibrous, 1/8" chopped                                                                 5   --  --  --  --  --  --  --  --                             Properties                                                                    Unnotched Izod Impact, ft.-lbs.,                                                             7.36                                                                              8.3 17.4P                                                                             17.5P                                                                             19.6                                                                              --  --  15.2P                                                                             --                             ft.-lbs., 25° C.                                                                          P   4NB 1NB 4NB 5NB 5NB --  5NB                            Drop ball impact, ft. lbs.                                                    25° C.  35              88.0                                           -40° C. 9               25.9                                           __________________________________________________________________________     *Control; P = partial break; NB = no break; S = specimens used                .sup.a DAVISON, SYLOID 234                                                    .sup.b CABOT, CABOSIL M5                                                      .sup.c DAVISON, Silica gel                                                    .sup.d CABOT, CABOSIL N70TS                                                   .sup.e IMSIL, IMSIL A 108                                                     .sup.f IMSIL, IMSIL A 108H                                                    .sup.g TULLANOX A50                                                           .sup.h CABOT, CABOSIL HS5                                                

The foregoing results demonstrate that highly impact resistant foams areprepared according to the present invention when a particulate foamnucleating agent comprising synthetic silica with an aspect ratio ofabout 1.0 is included. When, on the other hand, fibrous glass ispresent, impact strenth is much lower, and at -40° C. the ductile impactstrength is vastly inferior to, for example, the composition of Example4, foamed. The cells in the foams of Examples 1-8 are substantiallyuniform and void-free, as well, whereas the cells in Example 1A arenon-uniform and the foamed article contains numerous voids.

EXAMPLES 9

The general procedure of Examples 1-8 is repeated, substituting for thesilica, a non-siliceous inorganic particulate material in particulateform. The composition used and the results obtained are set forth inTable 3.

                  TABLE 3                                                         ______________________________________                                        Foamed Compositions of Polycarbonate and                                      Particulate Non-Siliceous Materials                                           Example                  9                                                    ______________________________________                                        Composition (parts by weight)                                                 Poly(bisphenol A carbonate)                                                                            100                                                  5-phenyl-3,6-dihydro-1,3,4-                                                                            0.25                                                 oxadiazin-2-one blowing agent                                                 Titanium dioxide, coated.sup.a                                                                         0.5                                                  Properties                                                                    Unnotched Izod Impact,   17.5P                                                ft.-lbs., 25° C.  2NB                                                  Drop ball impact, ft. lbs.                                                     25° C.           80                                                   -40° C.           25                                                   ______________________________________                                         .sup.a 0.15 micron particle size                                         

Impact resistant foams according to this invention are obtained. Flowwas difficult.

EXAMPLES 10-17

The general procedure of Examples 2 and 4 was repeated except that 0.2parts of various surfactants were added to each. The formulations usedand the results obtained are set forth in Table 4:

                                      TABLE 4                                     __________________________________________________________________________    Foamed Compositions of Polycarbonate,                                         Particulate Silica and Surfactants                                                              Example                                                                       10  11  12  13  14  15  16  17                              __________________________________________________________________________    Composition (parts by weight)                                                 Poly(bisphenol-A carbonate)                                                                     100 100 100 100 100 100 100 100                             5-phenyl-3,6-dihydro-1,3,4-                                                                     0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                                                                              0.25                            oxadiazin-2-one blowing agent                                                 fluorocarbon surfactant.sup.c                                                                   --  0.2 0   0   0   0.2 0   0                               Hydrophobic fumed silica.sup.a                                                                  0.5 0.5 0.5 0.5 --  --  --  --                              Hydrophilic fumed silica.sup.b                                                                  --  --  --  --  0.5 0.5 0.5 0.5                             Silicone fluid surfactant.sup.d                                                                 --  --  0.2 --  --  --  0.2 --                              Anionic polyglycol ether surfactant.sup.e                                                       --  --  --  0.2 --  --  --  0.2                             Properties                                                                    Unnotched Izod Impact,                                                                          17.4                                                                              19.6                                                                              16  --  17.4                                                                              17.7                                                                              --  19.6                            ft. lbs., 25° C.                                                                         4NB 3NB     5NB 4NB 2NB 5NB 1NB                             Drop ball impact, ft. lbs.                                                    25°  C.    73.8                                                                              ND  ND  82.6                                                                              ND  ND  ND  ND                              -40° C.    25.9                                                                              ND  ND  8.2 ND  ND  ND  ND                              __________________________________________________________________________     .sup.a CABOT, CABOSIL N 70TS                                                  .sup.b CABOT, CABOSIL M5                                                      .sup.c DuPont Co., ZONYL FSN                                                  .sup.d Dow Corning, Silicone fluid 200                                        .sup.e Sandoz Co., SANDOZIN D100                                         

The surfactant-containing compositions produced high impact strengthfoams with generally larger cell sizes.

EXAMPLES 18-22

The general procedure of Example 4 was repeated, except that severalwax-like materials were added as flow modifiers. The formulations usedand the results obtained are set forth in Table 5:

                  TABLE 5                                                         ______________________________________                                        Foamed Compositions of Polycarbonate,                                         Particulate Silica and Flow Modifiers                                         Example           18     19     20   21   22                                  ______________________________________                                        Composition (parts by weight)                                                 Poly(bisphenol-A carbonate)                                                                     100    100    100  100  100                                 5-phenyl-3,6-dihydro-1,3,4-                                                                      0.5    0.5    0.5  0.5  0.5                                oxadiazin-2-one blowing agent                                                 hydrophobic fumed silica.sup.a                                                liquid polyethylene.sup.b                                                                       --      0.3   --   --   --                                  solid polyethylene.sup.c                                                                        --     --      0.3 --   --                                  primary methyl behenate.sup.d                                                                   --     --     --    0.3 --                                  linear low density polyethylene.sup.e                                                           --     --     --   --    2.0                                Properties                                                                    Drop ball impact strength,                                                    ft.-lbs.  25° C.                                                                         88.0   68.3   67.8 86.9 85.0                                -40° C.    25.9   39.8   28.0 24.8 31.8                                ______________________________________                                         .sup.a CABOT, CABOSIL N 70TS                                                  .sup.b BARECO DIV., PETROLITE CO., VYBAR 825 P                                .sup.c BARECO DIV., PETROLITE CO., HYPAC Q 507                                .sup.d WITCO CHEM. CO., KEMESTER 902790                                       .sup.e UNION CARBIDE CO., BAKELITE 7047                                       ND = not determined                                                      

Certain of the modifiers help impact strength. Ethylene bis stearamideand an ester wax degraded during processing and could not be used.

EXAMPLES 23-27

The general procedure of Example 4 was repeated, including in thecompositions amounts of various impact modifiers. The formulations usedand the results obtained are set forth in Table 6:

                  TABLE 6                                                         ______________________________________                                        Foamed Compositions of Polycarbonate,                                         Particulate Silica and Impact Modifiers                                       Example          23      24     25   26   27                                  ______________________________________                                        Composition (parts by weight)                                                 Poly(bisphenol A carbonate)                                                                    100     100    100  100  100                                 5-phenyl-3,6-dihydro-1,3,4-                                                                    0.25    0.25   0.25 0.25 0.25                                oxadiazin-2-one blowing agent                                                 hydrophobic fumed silica.sup.a                                                                 0.5     0.5    0.5  0.5  0.5                                 Butadiene/styrene block                                                                        --       10    --   --   --                                  copolymer.sup.b                                                               Polycarbonate/polisiloxane                                                                     --      --      10   5   --                                  block copolymer.sup.c                                                         Acrylic rubber core shell                                                                      --      --     --   --    10                                 copolymer.sup.d                                                               Properties                                                                    Drop ball impact strength,                                                    ft.-lbs.,  25° C.                                                                       88.0    86.3   74.7 88.8 59.6                                -40° C.   25.9    23.8   53.8 44.8 25.9                                ______________________________________                                         .sup.a CABOT, CABOSIL N 70--T5                                                .sup.b SHELL, KRATON G 1651                                                   .sup.c General Electric Co., LR resin 3220                                    .sup.d Rohm & Haas Co., ACRYLOID KM 330                                       ND = not determined                                                      

Foamed compositions according to this invention are obtained. When abutadiene based core shell polymer was added, the processingtemperatures caused it to degrade. The block polycarbonate/polysiloxanecaused a very substantial improvement in low temperature impactstrength.

EXAMPLES 28-29

The general procedure of Example 4 was repeated and pigments and flowmodifiers were added. The formulations employed and the results obtainedare set forth in Table 7:

                  TABLE 7                                                         ______________________________________                                        Pigmented Foamed Compositions of                                              Polycarbonate and Particulate Silica                                          Example           B*     10     20   28   29                                  ______________________________________                                        Composition (parts by weight)                                                 Poly (bisphenol A carbonate)                                                                    100    100    100  100  100                                 5-phenyl-3,6-dihydro-1,3,4-                                                                      0.25  0.25    0.25                                                                               0.25                                                                               0.25                               oxadiazin-2-one blowing agent                                                 hydrophobic fumed silica.sup.a                                                                  --     0.5    0.5  0.5  0.5                                 carbon black pigment 0.2                                                                        --     --     0.2  0.2                                      solid polyethylene flow                                                                         --     --     0.3  --   0.3                                 modifier.sup.b                                                                Glass fibers      5.0    --     --   --   --                                  Titanium dioxide  0.5    --     --   --   --                                  Properties                                                                    Drop ball impact, ft.-lbs.                                                     25° C.     35    88.0   68   85.4 74.2                                -40° C.     9     25.9   28   26.8 29.6                                ______________________________________                                         *Control                                                                      .sup.a CABOT, CABOSIL N 70TS                                                  .sup.b Bareco Co., HYPAC Q 507                                           

The use of carbon black is not seen to adversely affect the impactstrength of the present invention. The foamed product of Example 31 isshown in FIG. 2. The foamed product of Example 1A is shown in FIG. 1. Incontrast to Example 1A, Example 31 produces uniform cells.

The foregoing patents, applications, and publications are incorporatedherein by reference. Many variations will suggest themselves to thoseskilled in this art in light of the above, detailed description. Forexample, the polycarbonate can be replaced in part by poly(ethyleneterephthalate), poly(1,4-butylene terephthalate)poly(1,4-cyclohexyldimethylene)terephthalate, and the like. The blowingagent 5-phenyl-3,6-dihydro-1,3,4-oxadiazinedione can be replaced with5-phenyl tetrazole. All such obvious variations are within the fullintended scope of the appended claims.

We claim:
 1. A foamable thermoplastic composition comprising(a) anaromatic polycarbonate, an aromatic polyester carbonate, an aromaticdihydric phenol sulfone carbonate, or a mixture of any of the foregoing,alone, or in combination with (b) a saturated polyester resin comprisingunits of an aliphatic diol, a cycloaliphatic diol, or a mixture of saiddiols and an aromatic diacid or a cycloaliphatic diacid, (c) a minoreffective amount of a chemical foaming agent; and (d) a minor effectiveamount of siliceous particulate foam nucleating agent which is coatedwith a siliceous compound to render that agent hydrophobic, alone, orcombined with a particulate organic polymeric material, said nucleatingagent being substantially insoluble in the composition and having anaspect ratio of not greater than about 1.0, the amounts of (c) and (d)being sufficient to provide a rigid substantially uniform cellular corewithin a solid integral skin.
 2. A composition as defined in claim 1which is substantially free of fillers having an aspect ratio of greaterthan about 1.0.
 3. A composition as defined in claim 1 wherein saidfoaming agent (c) is present in an amount of from about 0.02 to about2.0 parts by weight based on the 100 parts by weight of the totalcomposition.
 4. A composition as defined in claim 1 wherein said foamnucleating agent (d) is present in an amount of from about 0.01 to about40 parts by weight per 100 parts by weight of the total composition. 5.A composition as defined in claim 1 wherein foam nucleating agent (d) isselected from amorphous silica, precipitated silica and fumed silica,alone or together with an organic polymeric particulate material.
 6. Acomposition as defined in claim 5, in which the particulate organicpolymeric material is selected from acrylate copolymers, polyphenyleneether resins and polytetrafluoroethylenes.
 7. A composition as definedin claim 6, in which the particulate organic polymer is an acrylatecopolymer.
 8. A composition as defined in claim 7, in which the acrylatecopolymer is an acrylate-based coreshell multi-phase compositeinterpolymer resin.
 9. A composition as defined in claim 8, in which thecore-shell interpolymer comprises about 25 to 95 percent by weight of afirst elastomeric phase and about 75 to 5 percent by weight of a finalrigid thermoplastic shell phase, and optionally one or more intermediatephases.
 10. A composition as defined in claim 9, in which the core-shellinterpolymer has been polymerized from a C₁ to C₆ alkyl acrylateresulting in an acrylic rubber core crosslinked with a crosslinkingpolyethylenically unsaturated monomer and further containing agraftlinking monomer which is also polyethylenically unsaturated.
 11. Acomposition as defined in claim 10, in which the alkyl acrylate is butylacrylate.
 12. A composition as defined in claim 6, in which theparticulate organic polymer is a polyphenylene ether resin.
 13. Acomposition as defined in claim 12, in which the polyphenylene etherresin is poly(2,6-dimethyl-1,4-phenylene ether).
 14. A composition asdefined in claim 6, in which the particulate organic polymer ispolytetrafluorethylene.
 15. A composition as defined in claim 1 whichalso includes a surface active agent compound comprising a silicone, apolyglycol ether, a fluorocarbon, a fatty acid derivative or a mixtureof any of the foregoing.
 16. A composition as defined in claim 1 whichalso includes (e) an effective amount of a modifier to increase the flowof the composition during molding.
 17. A composition as defined in claim8 wherein said modifier (e) is selected from waxes, fatty acid esters,polyethylene waxes or a mixture of any of the foregoing.
 18. Acomposition as defined in claim 1 which also includes (f) an effectiveamount of a modifier to increase the impact strength of the composition.19. A composition as defined in claim 1 wherein component (a) comprisesan aromatic polycarbonate.
 20. A composition as defined in claim 12wherein component (a) comprises poly(bisphenol A) carbonate.
 21. Acomposition as defined in claim 1 wherein component (c) comprises achemical compound which can be decomposed to release a gas harmless toresin components (a) and (b).
 22. A composition as defined in claim 21wherein component (c) comprises a nitroso compound, a semicarbazidecompound, a tetrazole compound, an oxalate compound, a triazinecompound, a dihydrooxadiazinone compound, or a mixture of any of theforegoing.
 23. A composition as defined in claim 22 wherein component(c) comprises 5-phenyl-3,6-dihydro-1,3,4-oxadiazin-2-one.
 24. Acomposition as defined in claim 1 wherein component (d) comprises asynthetic, non-crystalline silica.
 25. A composition as defined in claim24 wherein component (d) has an average particle size in the range of0.5 to 50 microns.
 26. A composition as defined in claim 25 whereincomponent (d) has an average particle size in the range of 0.008 to0.020, microns.
 27. A composition as defined in claim 26 whereincomponent (d) is hydrophobic.
 28. A foamable thermoplastic compositioncomprising(a) poly(bisphenol A carbonate); (c) from about 0.02 to about2.0 parts by weight of a dihydrooxadiazinone foaming agent, and (d) fromabout 0.1 to 5.0 parts by weight of a particulate, synthetic,non-crystalline hydrophobic silica which is coated with a siliceouscompound to render that silica hydrophobic, having an average particlediameter of less than about-10 microns, the parts by weight being basedon 100 parts by weight total of (a), (c) and (d).
 29. An article ofmanufacture comprising a foamed composition as defined in claim
 1. 30.An article of manufacture formed by foaming a composition as defined inclaim 29, said article comprising a rigid, substantially void-free,substantially uniform cellular core within a solid integral skin.